iio: adc: ad7606: add gain calibration support

Add gain calibration support, using resistor values set on devicetree,
values to be set accordingly with ADC external RFilter, as explained in
the ad7606c-16 datasheet, rev0, page 37.

Usage example in the fdt yaml documentation.

Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250606-wip-bl-ad7606-calibration-v9-7-6e014a1f92a2@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>